Communications system



Examiner 5 Sheets-Sheet l w. s. HALsTl-:AD

Filed Aug. 11, 1944 COMMUNICATIONS SYSTEM Nov. 26, 1946.

Examine 5 Sheets-Sheet 2 W/L/AM 5. HALTf/ID INVENTOH ATTORNEY A Examnel`935 Nov. 26, 1946. w. s. HALsTl-:AD 2,411,786

COMMUNICATIONS SYSTEM Filed Aug. ll, 1944 5 Sheets-Sheet 3 l n 9 m W/LL/A M 5. HALSTEAD INVENTOR BY AW ATI'OHNEY Examm U9, ELEPHUm.

NOV- 26, 1946- w. s. HALSTEAD COMMUNICATIONS SYSTEM Filed Aug. 11, 19445 Sheets-Sheet 4 D r f am/.MM Hw A 5. /1 M, M u W if Hom,

Nov. 26, 1946. w. s. HALSTEAD 2,411,786

COMMUNICATIONS SYSTEM Filed Aug. 11, 1944 5 sheds-sheet 5 f l 250] 257)f4? f MAsTm A pon/fp 1 asc/Lum@ 4m/Hf@ v 24s 248 f AUD/0 AMPL/F/QW/LL/AM 5. HALSTEAD HVVENTOR ATToRN'gY A U9, ltLtvhum Patented Nov. 26,1946 Examiner,

COMMUNICATIONS SYSTEM William S. Halstead, Purchase, N. Y., assigner,

by mesne assignments, to Farnsworth Television & Radio Corporation, FortWayne, Ind., a

corporation of Delaware Application August 11, 1944, Serial No. 548,962

12 claims. 1

This invention pertains to carrier wave communications systems ingeneral, and more particularly to a system for utilizing afrequencymodulated transmitter at a central station for signaldistribution over relatively large areas by means of public utilitieslines or other conductors, and receiving said signals in predeterminedsignal areas or zones on frequency-modulated zone receivers, andre-transmitting the signals at each particular localized signaling zoneon an amplitude-modulated zone transmitter.

By utilizing the system of the present invention, the high level ofnoise interference ordinarily encountered on power lines and otherconductors may be substantially eliminated because the frequencymodulation employed at the central station has a greatly improved signalto noise ratio over that provided by an amplitudemodulated system.

My Patent No. 2,389,257, dated Nov. 10, 1945, for a Carrier Wavesignaling system, discloses in general a system utilizing power lines,wayside conductors, or any available conductors in general forestablishing a distributing network so that a central stationtransmitter may be used in conjunction with repeater stations. Thecentral station in the above-mentioned patent relies generally onutilizing a central transmitter operating on a comparatively lowfrequency while the repeater station receives the signalling impressedupon the distributing network and redistributes it in localized zones ona different frequency, such as on frequencies ordinarily incorporated inthe broadcast spectrum.

The present invention, however. differs in general from that of saidpatent in that the present invention employs frequency modulation at thecentral station transmitter, the signals of said transmitter beingreceived in localized signaling zones and re-transmitted onamplitude-modulated transmitters so that the conventional home typereceivers may be employed for receiving signals which originated at thecentral station as frequency modulated signals.

Throughout the application the term distributing network or waysideconductors is understood to mean any or all types of conductors such aspower lines, telephone or telegraph lines, cables, rails, pi sul bleconductor which.. permits a e operation of the m.

It is an object of the present invention to provide a carrier wavecommunication system utilizing power lines, telephone lines, telegraphlines, or any other suitable conductors for establishing communicationscoverage over a relatively large area, wherein a central station ormaster transmitter impresses frequency-modulated signals on power linesor other conductors. The frequency-modulated signals may be received inpredetermined signaling zones or areas on frequency modulation types ofreceivers, the outputs of which are impressed upon local zonetransmitters which in turn impress amplitudemodulated signals onlocalized conductors (which may or may not be part of the conductorsystem over which the frequency modulated signals are carried) forestablishing local signaling zones or areas.

A further object of the invention is to provide a system of the classdescribed wherein frequency-modulated signals are impressed upon publicutility lines or other wayside conductors which embrace a relativelylarge area, and having said signals received on frequency-modulated zonereceivers and retransmitted on amplitude-modulated zone transmitters inlocalized zones, with the frequency of adjacent amplitudemodulated zonetransmitters being different from one another to restrict interzoneinterference.

A further object of the invention is to provide various means ofcoupling a central station or master transmitter to a distributingnetwork embracing a relatively large area so that theamplitude-modulated transmitters of repeater stations may retransmit,over localized areas a signal which originally was transmitted from thecentral station on a frequency-modulated transmitter. By this meansconventional amplitudemodulated broadcast receivers may be employed forreceiving intelligence which originated at the central contact stationas frequency-modulated signals. 4

A further object of the invention is to provide a repeater boostercomprising a receiver and transmitter with a relay control system whichis employed at a point on the power lines or other conductors wherethere is appreciable attenuation, said attenuation usually being causedby excessively long lines or caused when said conductors run for aconsiderable distance underground or through metal conduit.

Further and other objects may be and may become apparent from a perusalof the present disclosure, since the present showing is by way ofillustration only and is not to be construed as a limitation except asdefined by the supporting claims.

In the drawings:

Fig. 1 is an overall block diagram of an induction radio system.

Fig. 1A is a modification of Fig. 1, showing an alternative method ofcoupling the equipment to the distribution network.

Fig. 2 is a block diagram of a frequency-modulated transmitter.

Fig. 3 is a schematic diagram of a frequencymodulated transmitter asrepresented in block diagram Fig. 2.

Fig. 4 is a block diagram of a frequency modulation zone receiver havingmeans for coupling to a zone transmitter.

Fig. 5 is a schematic diagram of the frequency llrpiiodillated zonereceiver shown in block diagram Fig. 6 is a block diagram of anamplitude modulated zone transmitter.

Fig. I shows the schematic diagram of the ampiitude modulatedtransmitter shown in Fig. 6.

Referring to the drawings, and more particularly to Fig. 1 thereof, thenumeral 8 designates a high voltage line, as for instance a 23.000 voltline, on which is a 100 kilocycle frequency modulated carrier and 23.000volt feeders extending throughout the service zone.

A microphone I5 or dual turntable with suitable amplifiers |4 isconnected to a 100 kilocycle frequency modulated central stationtransmitter I3. Said transmitter is coupled to the 115 volt line I2 andfeeds through its associated transformer II, 2300 volt line I0 and thenthrough transformer 9 into the 23000 volt feeders 8 extending throughoutthe service area.

There are here shown zones I and 2, and as the description of one zonewill sumce for both or any duplication thereof, the apparatus in zone Iwill be described.

The transformers I6 and I8 of zone I connected by lines |1 and I9 feedthe 100 kilocycle FM carrier to the 100 kilocycle FM zone receiver 20.The output of zone receiver 20 is coupled to a 590 kilocycle amplitudemodulated zone transmitter 22. This zone transmitter 22 is automaticallycontrolled by a FM carrier operated relay circuit 23. which operates thetransmitter 22 whenever the central station transmitter I3 is inoperation. The output of said transmitter 22 is coupled back into thepower lines I9, I1, and 26 through its associated transformers |8 and 25into a standard broadcast receiver 21, any number of which similarreceivers can be operated on various sub-circuits in the zone.

In Figs. 2 and 3, the central station transmitter is shown in greaterdetail, Frequency-modulation is employed in this transmitter and also inthe zone receivers in order to minimize disturbances, due to noise.

As will be seen in the block diagram Fig. 2, a microphone 21, which maybe any type of signal originator, is connected to a reactance tubemodulator 28, which is coupled to an oscillator 29, :iid a poweramplifier 30, attached to an antenna In Fig. 3, this apparatus is setforth in greater detail and may be taken as indicative of one form thatthis may assume. In this figure oscillator tube 34 is coupled throughvariable resistor 82 and capacitor 42 supplying radio frequency energyto the reactance modulator tubes 32 and 33 and their associated circuitwhich functions in a conventional manner when a modulated input isimpressed at input 15. Oscillator tube 34 is also coupled throughcoupling capacitor 4.1 t0

power amplier tube 35 and its associated circuit. 'I'he power amplifiertank circuit consists of in ductance 64 tuned by the variable capacitor52 which is in turn shunted by xed capacitors such as 53, 54, 55, and56. These xed capacitors may be cut in or out of the circuit for tuningby means of switch 94. Inductance 65 is inductively coupled toinductance 64 and couples to the 115 volt power line through attenuator90 and capacitors 51 and 58, at terminals 88 and 89, or these terminals88 and 89 can be connectedto a radio frequency transmission line 9|,said line to run parallel and in close proximity to a power line 92 andending in a terminating unit 93. 'Ihis latter method of coupling isshown in Fig. 1A and description thereof. Voltage regulator tubes 36 arerequired for stable operation of tube 32, 33, and 34.

Rectifier tube 38 is a full wave rectier which with its associated powertransformer 12 supplies high voltages to the plates and screen grids ofthe transmitter tubes. Filter chokes 92 and 93 in conjunction with ltercapacitors 59, 60, and

. 6| produce a smooth direct current output from the power supply.

The frequency modulated zone receiver is illustrated in Figs. 4 and 5,particular reference being made to Fig. 4, where the same is shown inblock diagram, consists of an antenna 95, a R. F. ampliner 96, aconverter 91, an I. F. amplifier 98, a limiter 99, a discriminator |00,and an A. F. amplier |0I, which leads to the amplitude-modulatedtransmitter. Leading from the limiter 99, is a D. C. amplifier |02operating relay |03, which in turn controls said amplitude modulatedtransmitter when the central station transmitter is in operation.

The detailed circuit as shown in Fig. 5 is for the most part aconventional superheterodyne receiver except for the addition of limitertubes |09 and ||0 and discriminator tubes III and |I2 and consists oftwo radio frequency tube stages |04 and |05, a converter tube |06,followed by two intermediate frequency tubes |01 and |08. The signal isthen applied to the cascade limiter consisting of tubes |09 and I|0 sothat the amplitude of the applied signal is materially reduced therebysuppressing noise disturbances to a large degree. The FM signals arethen applied to a phase-shift type of discriminator, tubes and ||2 andassociated circuit Where the frequency deviations are converted intoamplitude variations. The resulting audio frequency signal is thenamplified by tube I3 and fed through transformer 228 where it then goesto the output receptacle 244, Receptacle 244 connects to the amplitudemodulated zone transmitter. Gain control 2|5 regulates the volume of thesignal to the desired amount. A conventional power supply and lter withtube I6 supplies plate and heater current for this receiver.

As a result of the limiter actionvoccurring in tube |09 a D.C. voltageis developed across resistor 202. This negative voltage is applied to aD.C. amplifier comprising tubes ||4 and ||5. The output of tube I|5 isconnected to relay 240 which leads to output terminals 24|. Contacts onrelay 240 control the plate voltage supply of the A. M. transmitter.Relay 240 operates only when suflicient negative voltage is obtainedfrom resistor 202 and this occurs only when the carrier frequency of theFM central station transmitter is being received.

Fig. 6 is the block diagram of the amplitude modulated zone transmitteroperating on some 179, iELEPHONY.

Examiner established frequency in the range limit of standard broadcastreceivers employed, and usually at a predetermined frequency found to besignal free, and in this instance 590 kilocycles. In this instance, forexample, master oscillator 250 is amplified by power amplifier 25| andthe R. F. output is then fed to the antenna 241. The power amplier 25|is modulated by audio amplifier 249 which can be operated by any type ofsignal source such as microphone 248 or connected to the output terminal244 of the frequency modulated zone receiver described in Fig. 5.

In Fig. 7 is a detailed circuit of Fig. 6. Oscillator tube 252 is tunedby oscillator tank 253 and coupled to power amplifier tube 254 and itsassociated circuit through capacitor 255. The modulated output of thistube 254 is coupled through its tank inductance 253 to output couplinginductance 251. The modulating input 246 to the audio system is coupledthrough the speech input transformer 258 and the output of thistransformer connects to audio driver tube 259. A class B drivertransformer 260 couples tube 259 to the push-pull class B modulator tube26| which in turn connects to the modulation transformer 262. Thistransformer 262 supplies class B modulation to the power amplifier tube254.

A conventional power supply for the transmitter consists of platetransformer 263, rectifier tube 264, filter chokes 265 and 266 andfilter capacitors 261 and 268. Heater current for all tubes is suppliedby transformer 269. Switch 210 controls the input from the 115 volt lineto the power supply while switch 280 controls the operation of the platesupply transformer. Terminals 212 permit the transmitter to becontrolled from output terminals 24| controlled by relay 240 of Fig. 5,so that signals from the 100 kilocycle central station transmitter willbe re-broadcast automatically.

In operation, this amplitude-modulated zone transmitter has itsfrequency determined by setting the master oscillator tuning condenser213, the proper setting being determined from the calibration curve.With the R. F. output disconnected, the power amplifier coarseadjustment switch 214 is rotated until a dip is obtained on the platemilliameter 28|. The power amplifier fine adjustment tuning condenser215 is then adjusted to give a minimum defiection on the platemilliameter 28|. The R. F. output 216 is plugged into a 115 volt sourcewith the R. F. output attenuator 211 set approximately in half-wayposition. Then the coupling is varied between amplifier tank 256 andoutput coil 251 until a maximum desired deection is obtained on theplate meter 28|, or output terminals 216 may be coupled to a tunedtransmission line as disclosed in Fig. 1A. The audio-frequencyattenuator 219 is now adjusted so that the modulation indicator 218fiashes only on severe peaks, and should the indicator 218 remainlighted, there will be severe distortion.

Fig. 1A shows an alternate method of applying induction radio signalingto a power line circuit. Microphone I5 or dual turntable with amplifiersI4 is connected to a 100 kilocycle frequency modulated central stationtransmitter I3. The -output from transmitter I3 is then fed into a tunedline |20 which extends for a distance of from 500 to 1000 feetsubstantially parallel and in close proximity to a conventional 2300volt primary power circuit |22. Tuned line |20 is terminated at itsextreme end by a suitable terminating unit 2| to avoid standing waveradiation, substantially conning the signals to the power line and itsimmediate field.

The illustration indicates that the circuit |22 may run for a distancein underground conduit |23 before coming to the surface at zone wherethe FM induction radio signals are picked up by receiving antenna |24which connects to zone receiver |21 as previously described andillustrated in Figs. 4 and 5. Zone receiver |21 in turn is connected tothe A. M. transmitter |28 and with identical relay control circuit |29as previously described in detail in reference to Fig. 5. Ihe A. M. zonetransmitter operating on 590 kilocycles is then fed into tuned line |30,said line being terminated by a terminating unit |3| to eliminatestanding waves in the same manner as described in reference to FMantenna |20. This line |30 is run parallel and preferably in closeproximity to the 2300 volt power. circuit for a distance of from 500 to1000 feet thereby imposing the 590 kilocycle A. M. signal back into thelocal zone circuit and its secondary power circuits such as circuit |32to which may be connected a conventional broadcast receiver |35. Anynumber of similar receivers can be used to pick up the signal on thisand other circuits in the zone. The broadcast receiver |35 picks up thesignal from the power circuit inductively by antenna |36.

From the foregoing descriptions taken in connection with the drawings,it should be evident that an induction radio signaling apparatus isprovided.

inasmuch as the transmission of the amplitude-modulated transmitter mustbe of a type that transmits only to a restricted zone, it will be notedthat by the adjustment of the several elements thereof, and moreparticularly the attenuator 219, which not only acts as a radiofrequency attenuator but also a line coupling device, such result isattained, and that only receivers within the restricted zone will beaffected thereby.

Also that it is specially useful in army camps, and similar places wherebroadcasting of a more or less secret nature is desirable. Also thatwith this system receiver sets of conventional types can be utilized inthe houses or homes within the selected area; and that with this systemthe signals are confined within such area for the system permitsestablishment of a purely localized broadcasting service in thebroadcast band with transmission of effective signals being coni-inedsubstantially within limits of, as for instance, the cam area. Thissystem thereby affords a relative degree of security in suchtransmissions as compared with those of normal space-radio systems andpermits, as before noted, of the use of conventional broadcastingreceivers in all buildings within the entire camp area which may extendmany square miles.

It will also be understood that a frequencymodulated central stationtransmitter sends out the broadcasting signals from a central studiolocated for example, at an army camp, then through the distributingnetwork extending throughout the camp or other area. The induction radiofield, extending laterally from these conductors reaches out for adistance of several hundred feet in actual use, dropping away so rapidlythat at a distance of 300 to 400 feet, the signal ordinarily is belowthe noise level. Also the signal intensity of' the induction field, asmeasured with a eld strength meter located at a given lateral distancefrom the conductors at the edgeof the selected area, may be regulated toa desired field strength value, and which in this country would bespecified by the Federal Communications Commissions regulations.

While certain specic illustrations of the invention are shown, it is tobe understood that they are merely illustrative of the invention andthat many other modifications may be employed without departing from thespirit of the invention as defined by the subjoined claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a carrier wave communications system, means for generating afrequency-modulated carrier, means for coupling said frequency-modulatedcarrier to a distributing network, means for coupling said network to areceiver responsive to frequency modulation, an amplitude modulatedtransmitter, means for coupling said frequency modulated receiver outputto an amplitude modulated transmitter operating on a frequency differentthan said first transmitter, means for coupling the output from theamplitude modulated transmitter back into said distributing network, asecond receiver operating in the local zone area of the distributingnetwork and tuned to the frequency of said amplitude modulatedtransmitter, and means for reproducing into sound the output of thesecond receiver.

2. In a restricted zone signaling system, means for generating frequencymodulated radio frequency signals, means for coupling, superimposing,and adjusting the frequency modulated radio frequency signals to apredetermined level on an electric transmission line, means for couplingsaid transmission line to a receiver located at a remote point, couplingmeans for re-impressing the received intelligence of said radiofrequency signals on a second transmitter, a second transmitter of theamplitude modulation type operating on a frequency different than saidfirst transmitter, 'means for coupling, superimposing and adjusting to apredetermined level the second frequency on said electric transmissionline, a receiver disposed within the restricted signaling zoneestablished by the second transmitter and adjusted to said secondfrequency, and means connected to the output of said receiver fortranslating the intelligence of said second frequency.

3. In a restricted zone signaling system, means for generating frequencymodulated radio frequency signals, means for coupling, superimposing,and adjusting the frequency modulated radio frequency signals to apredetermined level on an electric transmission line, means for couplingsaid transmission line to a receiver located at a remote point, couplingmeans for re-impressing the received intelligence of said radiofrequency signals on a second amplitude modulated transmitter, saidsecond amplitude modulated transmitter operating on a frequencydifferent than said first transmitter, the frequency between said secondtransmitter and said rst transmitter being such as to eliminateinteraction therebetween, means for coupling, superimposing andadjusting to a predetermined level the second frequency on said electrictransmission line, a receiver disposed within the restricted signalingzone established by the second transmitter and adjusted to said secondfrequency, and means connected to the output of said receiver fortranslating the intelligence of said second frequency.

4. In a restricted zone signaling system, means for generating frequencymodulated radio frequency signals, means for coupling, superimposing,and adjusting the frequency modulated radio frequency signals to apredetermined level on an electric transmission line, said radiofrequency being of such a value as to pass through line transformerswith minimum attenuation, means for coupling said transmission line to areceiver located at a remote point, coupling means for reimpressing thereceived intelligence of said radio frequency signals on a secondamplitudemodulated transmitter, said second amplitudemodulatedtransmitter operating on a frequency different than said firsttransmitter, the frequency difference between said second transmitterand said iirst transmitter being such as to eliminate an interactiontherebetween, means for coupling, superimposing and adjusting to apredetermined level the second frequency on said electric transmissionline, a receiver disposed within the restricted signaling zoneestablished by the second transmitter and adjusted to said secondfrequency, and means connected to the output of said receiver fortranslating the intelligence of said second frequency.

5. In a restricted zone signaling system, means for generating frequencymodulated radio frequency signals, means for inductively coupling saidfrequency modulated radio frequency signals to an electricallyconducting line of said system, a termination unit connected to saidlast means to restrict the formation of standing waves thereon, meansfor inductively coupling said electrically conducting line to areceiver, coupling means for re-impressing the received radio frequencysignals on a second amplitude modulated transmitter, said secondamplitude modulated transmitter operating on a frequency different thansaid first transmitter, means for inductively coupling the secondfrequency signals t0 an electrically conducting line of said system, atermination means connected to said last named means to restrict theformation of standing waves thereon, a receiver disposed within therestricted signaling zone established by said second transmitter andadjusted to said second frequency, and means connected to the output ofsaid receiver for translating the intelligence of said second frequency.

6. In a restricted zone carrier wave signaling system having at leastone electrically conducting line, a frequency-modulated transmitter fortransmitting modulated radio frequency signals, means for coupling saidmodulated radio frequency signals to an electrically conducting line ofsaid system, a plurality of frequency modulation receivers, means forcoupling said electrically conducting line of said system to saidplurality of receivers, a plurality of amplitude modulated transmitters,coupling means for re-impressing the received radio frequency signals onsaid plurality of amplitude modulated transmitters, one of saidplurality of amplitude modulated transmitters being complemental to oneof said plurality of receivers, each of said plurality of transmittersoperating on a frequency different than said first frequency-modulatedtransmitter, and means for coupling each of said complementaltransmitters to an electrically conducting line of said system.

7. In a centrally controlled carrier wave distribution system having atleast one electrically conducting line, a central station frequencymodulated transmitter, means for modulating said central frequencymodulated station transmitter, a radio frequency attenuator and linecoupling unit connected to the output of said transmitter, the output ofsaid radio frequency 179, lELEFHONY, Examnel attenuator and linecoupling unit being connected to said electrically conducting line, asecondary communications distributing system including a frequencymodulated zone receiver and an 10 termination means connected to thecoupling means to restrict the formation of standing waves on saidcoupling means, a plurality of frequency modulated zone receivers, meansfor couamplitude modulated zone transmitter, means pling` saidelectrically conducting line of said for automatically initiatingoperation of the system to said plurality of frequency modulatedamplitude modulated zone transmitter when the receivers, a. plurality ofamplitude modulated carrier from the frequency modulated zone rezonetransmitters, coupling means for re-impresceiver is impressed upon saidamplitude modulatsing the received radio frequency signals on ed zonetransmitter, and a zone radio frequency said plurality of amplitudemodulated zone transattenuator and line coupling unit connected tomitters, one of said plurality of amplitude modthe output of said zonetransmitter, the output ulated zone transmitters being complemental t0of said zone radio frequency attenuator and line one of said pluralityof amplitude modulated recoupling unit being connected to saidelectrically ceivers, each of said amplitude modulated zone conductingline to establish a localized amplitransmitters in adjacent zonesoperating on diftude modulated signaling zone. ferent frequencies, allof said amplitude modu- 8. In a centrally controlled carrier wavedislated zone transmitters operating on a frequency tribution systemhaving at least one electrically different than said frequency modulatedcentral conducting line, a frequency modulated central stationtransmitter, means for inductively coustation transmitter, means formodulating said pling each of said complemental amplitude modfrequencymodulated central station transmitter, ulated transmitters to anelectrically conducting a radio frequency attenuator and line couplingline of said system, and termination means conunit connected to theoutput of said transmitnected to each of last said coupling means toter, the output of said radio frequency attenurestrict the formation ofstanding Waves on each ator and line coupling unit being connected to oflast said coupling means. an electrically conducting line of saidsystem, a 1l. In a centrally controlled carrier wave dissecondarycommunications distributing system tribution system having at least oneelectrically including a frequency modulated zone receiver conductingline, a frequency modulated central and an amplitude modulated zonetransmitter, station transmitter, means for modulating said means forautomatically initiating operation of frequency modulated centralstation transmitter, the amplitude modulated zone transmitter when aradio frequency attenuator and line coupling the carrier from thefrequency modulated zone unit connected to the output of saidtransmitter, receiver is impressed upon said zone amplitude the outputof said radio frequency attenuator modulated transmitter, and a zoneradio freand line coupling unit being connected to said quencyattenuator and line coupling unit conelectrically conducting line, asecondary comnected to the output of said amplitude modulatmunicationsdistributing system including a freed zone transmitter, the output ofsaid zone radio quency modulated zone receiver and an amplifrequencyattenuator and line coupling unit betude modulated zone transmitter,means for autoing connected to an electrically conducting line maticallyinitiating operation of the amplitude of said system. modulated zonetransmitter when the carrier 9. In a restricted zone carrier Wavesignaling from the frequency modulated zone receiver is system having atleast one electrically conductimpressed upon said amplitude modulatedzone ing line, a frequency modulated central station transmitter, saidautomatic initiating means intransmitter for transmitting modulatedradio cluding a carrier-operated relay responsive to frequency signals,means for coupling said modoperation of said frequency modulation zonereulated radio frequency signals to an electrically ceiver, and a powersupply relay responsive to conducting line of said system, a pluralityof operation of said carrier-operated relay to supfrequency modulatedzone receivers, means for ply plate voltage for said amplitude modulatedcoupling said electrically conducting line of said zone transmitter, anda zone radio frequency atsystem to said plurality of frequency modulatedtenuator and line coupling unit connected to the receivers, a pluralityof amplitude modulated zone output of said amplitude modulated zonetranstransmitters, coupling means for re-impressing mitter, the outputof said zone radio frequency the received radio frequency signals onsaid pluattenuator and line coupling unit being connectrality ofamplitude modulated zone transmitters, ed to said electricallyconducting line to estabone of said plurality of amplitude modulatedzone lish a localized signaling Z011@- transmitters being complementalto one of said 12. In a communications system of the class plurality offrequency modulated receivers, each described utilizing a distributingnetwork, a freof said amplitude modulated zone transmitters quencymodulated master transmitter, means for in adjacent zones operating ondifferent frequenmodulating Said master transmitter, a distribcies, auof said zone amplitude modulated transo utins network, means forcoupling Said master mitters operating on a frequency different thantransmitter to said distributing network, a plusaid frequency modulatedcentral station transrality of frequency modulated zone receivers,mitter, and means for coupling each of said commeans fOl' Coupling SaidZone reCeVers to the plemental amplitude modulated transmitters todistributing network, a plurality of amplitude an electricallyconducting line of said system. modulated zone transmitters to operateon sub- 10. In a restricted zone carrier wave signaling stantiallyidentical frequencies, means for cousystem having at least oneelectrically conductpling each of said frequency modulated zone reingline, a. frequency modulated central station ceivers to a complementalamplitude modulated transmitter for transmitting modulated radio zonetransmitter, and means for coupling said frequency signals, means forinductively couzone transmitters to the distributing network.

pling said modulated radio frequency signals to an electricallyconducting line of said system,

WILLIAM S. HALSTEAD.

